Mechanisms of transplacental transmission of human cytomegalovirus (HCMV), the leading viral cause of congenital infection affecting 1-3% of births in the U.S., are unknown. Primary maternal infection in first trimester poses a 40-50% risk of transplacental transmission and permanent birth defects in 15% of diseased babies. We propose to study HCMV infection of the human placental progenitors of the trophoblast (TB) and hematopoietic lineages, and the consequences in terms of the molecular mechanisms that dysregulate development. The Fisher group identified the early-gestation human placental stroma as a niche for cells that co-expressed markers of pluripotency and determinants of mouse TB fate, which suggested that this region is one source of human TB progenitor cells (TBPCs). They isolated these cells and developed lines of continuously self-renewing TBPCs. When cultured under conditions that triggered TB differentiation, the cells formed the mature human TB populations-multi-nucleated, transport syncytiotrophoblasts (STBs) and invasive CTBs. In the same region, the Fisher group identified hematopoietic stem cells (HSCs) as well intermediate precursors of the myeloid- and erythroid-committed lineages, suggesting active hematopoiesis. Cultured in defined medium, the CD34++CD45low HSCs contributed to erythrocytes and myeloid cells. Together these studies showed that the human placenta is a source of TBPCs that populate the chorionic villi and HSCs that might play a role in development of the hematopoietic system of the embryo/fetus. It is likely that placental HSCs also contribute to the placental Hofbauer macrophage population. TBPCs are highly susceptible to infection with a pathogenic HCMV strain that induced expression of proteins controlling cell-cycle progression and the balance between self-renewal and lineage-commitment. Upregulated molecules were mislocalized to the cytoplasm and accumulated in the virion assembly compartment. Additional experiments showed that HCMV infected placentally-derived HSCs. Here, we propose testing the theory that HCMV infection alters the balance between TBPC self-renewal and differentiation (Aim 1). We also hypothesize that infection of HSCs impairs their ability to form myeloid- and erythroid-committed lineages (Aim 2). These studies will provide new insights into viral effects on differentiation of TBPCs, which carry out the specialized functions of the placenta, and HSCs, which contribute to fetal hematopoiesis and the placental immunological barrier. Then we will use this information to determine if the same phenotypic changes are observed in placentas that were congenitally infected in utero. We will also measure the ability of human neutralizing MoAbs to protect chorionic villi from HCMV infection (Aim 3). In summary, these studies will provide new information about the effects of HCMV on TBPC and HSC self-renewal and differentiation. We also expect to gain insights into novel approaches for preventing HCMV transmission and the attendant birth defects.